Axon pathfinding depends on attractive and repulsive turning of growth cones to extracellular cues. Localized cytosolic Ca2+ signals are known to mediate the bidirectional responses, but downstream mechanisms remain elusive. Here, we report that calcium-calmodulin-dependent protein kinase II (CaMKII) and calcineurin (CaN) phosphatase provide a switch-like mechanism to control the direction of Ca(2+)-dependent growth cone turning. A relatively large local Ca2+ elevation preferentially activates CaMKII to induce attraction, while a modest local Ca2+ signal predominantly acts through CaN and phosphatase-1 (PP1) to produce repulsion. The resting level of intracellular Ca2+ concentrations also affects CaMKII/CaN operation: a normal baseline allows distinct turning responses to different local Ca2+ signals, while a low baseline favors CaN-PP1 activation for repulsion. Moreover, the cAMP pathway negatively regulates CaN-PP1 signaling to inhibit repulsion. Finally, CaMKII/CaN-PP1 also mediates netrin-1 guidance. Together, these findings establish a complex Ca2+ mechanism that targets the balance of CaMKII/CaN-PP1 activation to control distinct growth cone responses.

Axon guidance requires signal transduction of extracellular cues through the plasma membrane for directional motility. Here we present evidence that cholesterol- and sphingolipid-enriched membrane microdomains (lipid rafts) mediate specific guidance responses of nerve growth cones. Disruption of lipid rafts by various approaches targeting cholesterol or gangliosides selectively abolished growth cone attraction and repulsion in BDNF and netrin-1 gradients, respectively, without affecting glutamate-induced attraction. Interestingly, local raft disruption on one side of the growth cone in bath BDNF or netrin-1 produced opposite turning responses to that induced by the gradients. Raft manipulation also blocked Semaphorin 3A-induced growth cone repulsion, inhibition, and collapse. Finally, guidance responses appeared to involve raft-dependent activation of p42/p44 MAPK and ligand-induced receptor recruitment to lipid rafts. Together with the observation of asymmetric receptor-raft associations at the growth cone in guidance gradients, our findings indicate that localized signaling through membrane rafts plays a role in mediating guidance actions of extracellular cues on developing axons.

Developing axons are guided to their appropriate targets by environmental cues through the activation of specific receptors and intracellular signaling pathways. Here we report that gradients of pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide widely expressed in the developing nervous system, induce marked attraction of Xenopus growth cones in vitro. PACAP exerted its chemoattractive effects through PAC1, a PACAP-selective G-protein-coupled receptor (GPRC) expressed at the growth cone. Furthermore, the attraction depended on localized cAMP signaling because it was completely blocked either by global elevation of intracellular cAMP levels using forskolin or by inhibition of protein kinase A using specific inhibitors. Moreover, local direct elevation of intracellular cAMP by focal photolysis of caged cAMP compounds was sufficient to induce growth cone attraction. On the other hand, blockade of Ca 2ϩ , phospholipase C, or phosphatidyl inositol-3 kinase signaling pathways did not affect PACAP-induced growth cone attraction. Finally, PACAP-induced attraction also involved the Rho family of small GTPases and required local protein synthesis. Taken together, our results establish cAMP signaling as an independent pathway capable of mediating growth cone attraction induced by a physiologically relevant peptide acting through GPCRs. Such a direct cAMP pathway could potentially operate in other guidance systems for the accurate wiring of the nervous system.

No abstract

The plasma membrane of cells contains a variety of lipid and protein molecules that are often segregated and heterogeneously distributed in microdomains. Lipid rafts represent a generalized concept of membrane microdomains that are enriched in cholesterol and sphingolipids and, characteristically, resistant to cold detergent extraction. Lipid rafts have recently received considerable attention because they are thought to be involved in many cellular functions, in particular, signal transduction for extracellular stimuli. Many of these functions are also intimately related to the processes involved in neural development, including neurotrophic factor signaling and synaptic plasticity. Recent studies from our lab and others have indicated an important role for lipid rafts in axonal growth and guidance. Specifically, our data show that lipid rafts on the plasma membrane provide platforms for spatial and temporal control of guidance signaling by extracellular cues. In addition, lipid rafts may also function in other aspects of axonal growth and guidance, including spatial and temporal regulation of adhesion, cytoskeletal dynamics, and growth cone motility. Further elucidating how membrane rafts are involved in guided axonal growth would provide important insights into the intricate signaling mechanisms underlying neuronal wiring, which is fundamental for normal brain development and functional recovery after injury and diseases.